Alkaline exchange membranes or anion exchange membranes (AEMs) allow for the transportation of anions (e.g., OH−, Cl−, Br−) from the cathode to the anode in an electrochemical reaction. AEMs are a critical component of AEM fuel cells, where hydrogen and oxygen are used to generate electricity, with water as a byproduct. AEMs are also used in water electrolysis, where water is split into hydrogen and oxygen using electricity. In both AEM fuel cells and water electrolysis, hydroxide ions (OH−) are transported through the AEM, along with water molecules. AEMs may also be used, for example, in batteries, sensors, and as actuators.
Known AEMs are generally unsuitable for use in AEM fuel cells or water electrolysis. Many commercially-available AEMs are based on polystyrene, which is generally considered a poor choice for AEM fuel cells or water electrolysis.
Other AEM materials include polysulfones, poly(phenylene oxide)s, poly(phenylene)s, poly(benzimidazolium)s, poly(arylene ether ketone)s, and poly(arylene ether sulfone)s. These polymers contain an arylene ether linkage (—O—) in the mid-chain and a benzyltrimethyl ammonium group in the side-chain. This combination, however, has been found to be chemically unstable and to degrade easily under highly alkaline conditions. Known polyarylenes, in particular, will contain ether linkages in the polymer backbone because they are typically synthesized by basic condensation reactions between diol monomers and dihalide monomers, which produces hydrogen chloride as a byproduct.
In addition, the chloromethylation reaction involved in the manufacture of these polymers requires the use of toxic reagents, long reaction times, and extensive optimization to reach a desired degree of functionalization. Side reactions (e.g., gelation) frequently occur over prolonged reaction times, making it difficult to achieve an ion-exchange capacity (IEC) above 2.5 mequiv/g.